Some aerospace systems, such as the International Space Station (ISS) and the Space Transportation System (STS), or Space Shuttle, produce large volumes of telemetry which must be transmitted to terrestrial stations for use by mission controllers and vehicle health managers, among others. The transmission occurs over multi-path, multi-tiered networks which include nodes within and exterior to the space vehicles. Because bandwidth for transmitting the telemetry data is limited, various approaches to bandwidth compression have been attempted, one of which is the transmit-by-exception approach.
In a transmit-by-exception telemetry data transfer, the only data transmitted are those telemetry values which have undergone a significant change since they were last transmitted. This approach can provide substantial bandwidth compression. A resulting difficulty, however, is that the user on the ground cannot tell the difference between data that is not changing because it has not required transmission and data that is not changing because some portion of the network is malfunctioning. Network anomalies may be intermittent, making some telemetry faulty and leaving some telemetry valid. Unchanging data is ambiguous at the point of reception as to the cause for the lack of change.
The problem is exacerbated when the telemetry data is to be used in Integrated Vehicle Health Management (IVHM) systems. IVHMs assess telemetry data using diagnostic and prognostic software to support vehicle health maintenance. Experience has shown that ambiguous telemetry data may cause known IVHM algorithms to produce erroneous results.
One approach is to observe a constantly-changing telemetry data element, or counter, transmitted from the same source node as the transmit-by-exception data to be disambiguated, or target data. The target data may be evaluated as valid if the frequently-changing telemetry data element is seen to change over the time period when the target data was sent. The approach has several weaknesses. First, in a multi-path, multi-tiered communications network signal noise can cause false data values in the counter data, leading an algorithm to conclude that the counter is operating when, in fact, it is not operating. Thus, target data that is invalid may be erroneously seen as valid.
Second, counters that change at different rates on different nodes are utilized. The temporal resolution of a pairing disambiguation scheme for an individual node is the period of that node's counter minus the pulse width of the data bit. The temporal resolution of the disambiguation scheme for the network as a whole is the resolution of the slowest counter in the network. The temporal resolution for the network as a whole matters because IVHM systems need a series of complete “snapshots” of the vehicle system that give, as closely as possible, the state of the vehicle at particular times. If some of the data has gone bad without notice, the snapshots will be flawed and the IVHM system will reach an erroneous conclusion.
Designers and operators of large networks such as those used with ISS and STS often use their own commercial-off-the-shelf (COTS) equipment. Counters of different frequencies are inevitable, and the counter for a particular node may be the most reliably changing telemetry data element rather than the fastest-changing element. Also, the most rapidly updating piece of telemetry from one node may still be slower than the slowest telemetry from another node. Ideally, each node might be equipped with a high-speed clock, but this would quickly recreate the bandwidth-saturation problem that transmit-by-exception telemetry was designed to solve. Likewise, retrofitting each network node with a dedicated counter would be impractical. If one counter in the network operates at 0.1 Hz, it could be nearly 10 seconds before a problem was noticed. Disambiguation schemes with low temporal resolutions are problematic in IVHM systems, so the pairing approach is rejected.
Speed is important in disambiguating telemetry. Raw telemetry arrives for processing in a continuous sequence of batches and a first batch should be completely disambiguated before the next batch arrives. A telemetry disambiguation system as disclosed in copending patent application Ser. No. 10/465,415, entitled METHOD AND APPARATUS FOR DISAMBIGUATING TRANSMIT-BY-EXCEPTION TELEMETRY FROM A MULTI-PATH, MULTI-TIER NETWORK and incorporated herein by reference, may use a data structure representing a network that may be searched for information regarding the status of nodes, paths, and links.
Accordingly, it is desirable to have a data structure representing a network that is rapidly accessible. It is also desired to use the data structure for purposes other than telemetry disambiguation, as a rapidly accessible data structure representing a network has many potential uses. In addition, it is desirable for the data structure to be easily updatable with network node status information. It is also desirable that the data structure be portable to different users. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.